Generally, a humidity sensor is used for various purposes, for example, in a hygrometer, a humidity sensor for cooking of food in a microwave oven, and the like. Examples of currently used humidity sensors include a capacitance type humidity sensor, a relative humidity sensor, and an absolute humidity sensor. The capacitance type humidity sensor is based on variation of dielectric constants by hygroscopic property of an organic material such as polyimide. The relative humidity sensor is based on resistance variation of a semiconductor ceramic, such as MgCr2O4. The absolute humidity sensor is based on a ceramic thermistor.
Of the humidity sensors, the absolute humidity sensor based on two thermistors is widely used as a humidity sensor for cooking of food in a microwave oven.
The absolute humidity sensor has an advantage in that it can stably detect the humidity because it is not susceptible to variation of a peripheral temperature.
The principles of humidity sensing of the absolute humidity sensor in the microwave oven are based on resistance variation by temperature variation of a thermistor as water vapor generated from food during cooking of food absorbs heat of the thermistor.
FIG. 1 shows a structure of a background art absolute humidity sensor. Referring to FIG. 1, two ceramic thermistors 1 and 2 coated with a passivation film such as a glass film are floating by being connected to a support pin 4 by a precious metal conductor 3, such as platinum. The ceramic thermistors 1 and 2 are packaged by a metal shield case 5 that isolates the two thermistors 1 and 2 from each other.
The thermistor 1 is exposed to the air to allow water vapor to be in contact with a surface of the thermistor 1 by means of a fine hole in the metal shield case 5. The thermistor 1 is used as a sensing element. The other thermistor 2 is sealed in a dry N2 atmosphere by the metal shield case 5 so as not to be in contact with the water vapor. The thermistor 2 is used as a reference element.
Therefore, if a bridge circuit consists of the two thermistors 1 and 2 and an external resistor, the water vapor generated from food during cooking of food absorbs heat of the thermistor 1 exposed to the air. Thus, resistance variation occurs in only the exposed thermistor 1. In this case, output variation occurs due to a bias voltage, thereby detecting the humidity.
Since the background art humidity sensor uses an element as a ceramic thermistor, heat capacity is great, and thus, sensitivity is low. Also, response time is slow and the size of the sensor becomes greater.
Furthermore, the thermistor element is floating using the conductor 3 and the support pin 4 as shown in FIG. 1, and the conductor 3 and the pin 4 are spot-welded. For assembly, the reference element 2 should be sealed in a dry N2. For this reason, the fabrication process steps are complicated and the number of the process steps increases. Also, the cost is expensive and mass production is disadvantageous.